1. Field of the Invention
The present invention generally relates to turbines and, more particularly, annular diffusers for the exhaust from such turbines.
2. Description of Related Art
With reference to FIG. 1, exhaust systems 1 of steam turbines and industrial gas turbines typically include an annular diffuser 3 and a collector 5. Annular diffuser 3 is arranged immediately downstream of the last turbine stage followed by a dump into collector 5. Collector 5 turns the exhaust gas 90 degree from a longitudinal axis of a shaft of the turbine. Such exhaust systems 1 are used to guide the flow from an exit plane of the turbine to a downstream component such as a condenser (not shown).
However, as illustrated in FIG. 1, the 90-degree in the collector distorts the flow field in the diffuser by causing mass flow to move toward the bottom and having much more diffusion near the top than on the bottom, thereby harming diffuser performance. Most attempts to mitigate this effect involve complex, expensive geometry that increases the axial length of the exhaust system.
For instance, U.S. Pat. No. 6,866,479 to Ishizaka et al. discloses an exhaust diffuser for use with an axial-flow turbine. The exhaust diffuser comprises a hub-side tube in a cylindrical shape located concentrically with a tip-side tube to form an annular flow passageway therebetween. The exhaust diffuser further includes front struts and rear struts placed axially at an interval in the exhaust diffuser. Additionally, U.S. Pat. No. 6,261,055 to Owczarek discloses an annular diffuser having its inlet located at the exit of the last turbine blade row. The diffuser is defined by an outer flow guide and an outer surface of a bearing cone. The outer flow guide extends from a casing of the turbine for 360 degrees circumferentially about a longitudinal axis of the turbine shaft. The bearing cone surrounds the turbine shaft. The diffuser provided by the combination of the outer flow guide and the outer surface of the bearing cone is in the form of an asymmetrical diffuser. However, each of the systems proposed by these patents result in complex, expensive geometries.
Accordingly, a need exists for an exhaust system for a turbine that provides improved pressure recovery while also providing little increase in complexity and manufacturing costs.